To the vehicle occupant a correctly fitting seat belt system is important in maximising both the comfort and convenience in normal use, and the dynamic performance in the event of an accident. A series of research projects has been carried out at MIRA to develop a mathematical model to aid vehicle designers in the configuration of seat belts with the aim of ensuring comfort and convenience to as great a percentage of the population as possible. This model is incorporated into a computer program called BELTFIT. The analysis technique takes into account any combination of anchorage locations, stalk characteristics, seat position and seat angles. The program predicts the belt paths on a wide range of occupant sizes and carries out a series of checks against comfort and safety criteria. Using dynamic programming techniques the path of the belt across the occupant is predicted by calculating the shortest route over the body surface. Rapid assessment of seat belt installations is possible without the need for lengthy subjective wearer trials. In addition to checks of fit on human wearers, the program can also be used to predict the fit on anthropomorphic test devices prior to impact test evaluations. This paper illustrates the development of the program and also describes the latest enhancements including restraint system design for rear seat occupants, which have been introduced with the aim of widening the application of the program.